Substantially water-insoluble compounds useful in imaging are generally incorporated into imaging materials as dispersions or emulsions. In many cases, the compound useful in imaging is dissolved in one or more organic solvents, and the resulting oily liquid is then dispersed into an aqueous solution containing, optionally, dispersing aids such as surfactants and/or hydrophilic colloids such as gelatin. Dispersal of the oily liquid into the aqueous medium is accomplished using high shearing rates or high turbulence in devices such as colloid mills, homogenizers, ultrasonicators, or homogenizers.
In the art of dispersion making, the use of organic solvents has traditionally been considered necessary to achieve small particle sizes, to achieve stable dispersions, and to achieve the desired reactivity of the compound useful in imaging. Some compounds that might be useful in unaging cannot be dispersed in the above manner, however, because of their poor solubility in most organic solvents. In other cases, the compound of interest may have sufficient solubility in organic solvents, but it may be desirable to eliminate the use of the organic solvent to reduce the attendant adverse effects, for example, to reduce coated layer thickness, to reduce undesirable interactions of the organic solvent with other materials in the imaging element, to reduce risk of fire or operator exposure in manufacturing, or to improve the sharpness of the resulting image. These and other disadvantages can be overcome by the use of solid particle dispersions in imaging as described in UK Patent No. 1,570,362 to Langen et al, U.S. Pat. No. 4,006,025 to Swank et al, U.S. Pat. No. 4,294,916 to Postle et al, U.S. Pat. No. 4,294,917 to Postle et al, and U.S. Pat. No. 4,940,654 to Diehl et al.
Techniques for making solid particle dispersions are very different from the techniques used to make dispersions of oily liquids. Typically, solid particle dispersions are made by mixing the crystalline solid of interest with an aqueous solution that may contain one or more stabilizers or grinding aids. Particle size reduction is accomplished by subjecting the solid crystals in the slurry to repeated collisions with beads of hard, inorganic milling media, such as sand, spheres of silica, stainless steel, silicon carbide, glass, zirconium, zirconium oxide, alumina, titanium etc., which fracture the crystals. The bead sizes typically range from 0.25 to 3.0 mm in diameter. Ball mills, media mills, attritor mills, jet mills, vibratory mills, etc. are frequently used to accomplish particle size reduction.
Unfortunately, the stabilization of solid particle dispersions is much more difficult than the stabilization of conventional liquid droplet dispersions, since traditional stabilizers such as anionic or nonionic alkyl or aryl surfactants tend to adsorb much more readily to liquid surfaces than to solid surfaces. In fact, the use of such traditional stabilizers in the making of solid particle dispersions frequently results in unwanted particle growth and/or needle-like crystal growth. Such particle growth is undesirable since it reduces the covering power of the compound in the coated layers of an imaging element, while the presence of needle-like crystals results in filter plugging and poor manufacturability. Water soluble polymers such as polyvinylpyrrolidone have been added to solid particle dispersions of sensitizing dyes to reduce particle or crystal growth, as described in U.S. Pat. No. 4,006,025 to Swank et al. However, sensitizing dyes dispersions are not the only solid particle dispersions useful in imaging and it is desirable to provide improved stabilized solid particle dispersions of other water-insoluble compounds useful in imaging.